Study On Key Techniques Of Profile Measurement For Large Aperture Optical Aspherics

The shape and surface figure measurement accuracy for large mirrors is mainlyrestricted by the circular axial run-out (CARO) error of the rotation spindle in addition tothe radial error of the rotation spindle, and the existing profile-measuring approaches forlarge mirrors can not satisfy requirements of high-precision, wide measurement range andnon-contact measurement, so the single-step spatial rotation error separation technique(SSEST) is introduced into the surface profile measurement and the method is improved tofulfill the demand of autonomous error separation. Moreover, with the non-contact laserdifferential confocal probing technique and the method of axis rotation metrology, a novelSSEST-based profile-measuring instrument for large mirrors is developed, and the involvedprinciples, measuring methods and key techniques are studied.The main creative contents are as follows:To further improve accuracy of surface profile measurement for large spherical andaspheric mirrors, a novel SSEST is proposed to separate the surface profile error andspindle spatial rotation error by using Discrete Fourier Transform and harmonic analysis,and the effectiveness of SSEST is verified by theoretical analyses and experiment.Furthermore, based on the SSEST, a high-precision air-bearing spindle which is consider asrotary datum for profile measurement of large mirrors is constructed, and performance ofthe developed spindle is tested by a series of experiments. The results show that the radialmotion error of the developed spindle is better than15nm within1-50upr (undulations perrevolution), and the axial motion error of the developed spindle is better than40.5nmwithin1-100upr. All these studies lay the foundation for the autonomous separation ofspindle spatial rotation error in the surface profile measurement for large mirrors.A novel automatic centering and leveling technique based on the aerostatic bearingtechnology is invented. In the method of centering, a planar aerostatic bearing and twomicro-displacement actuators are utilized to achieve centering operation, and in the levelingmethod, a spherical aerostatic bearing, two micro-displacement actuators and a spring pivotare employed to realize the leveling operation. Based on the technique, a turntable for theposition and posture adjustment of large surface under test (SUT) is developed, and the mathematical models of centering and leveling operation are obtained using coordinatetransformation, and coupling between the centering and leveling operation is also analyzed.Furthermore, by using distance-measuring interferometer and autocollimator, theresolutions of centering and leveling operation are measured. The experimental resultsshow that, with50Kg load, the leveling operation resolution is better than1.2″, levelingoperation range is±1.5°, the centering operation resolution is better than0.3μm, centeringoperation range is about±5mm.The zero-crossing triggering surface profile measuring technique which is based on thelaser differential confocal microscopy is researched, and a novel non-contact sensor basedon the technique is developed. The axial response property of the developed sensor whenmeasuring surface profile of large mirrors is obtained by simulation analysis andexperimental verification, and the resolution and repeatability of the developed sensor arealso obtained by series of experiments. The zero-crossing triggering surface profilemeasuring technique provides high-resolution and non-contact detection system for thedeveloped surface profile measuring instrument. The experimental results show that,sensitivity of the developed sensor is4.934V/μm, measurement range is250μm, theresolution is about5nm, and the repeatability is about20nm.Based on the above-mentioned key techniques, we study on the scanningmeasurement method of surface profile for large mirrors, and develop a novel surfaceprofile measuring instrument. Moreover, the vector model is established to analyze themotion error and misadjustment.According to the relevant regulations of verification, major technical specifications ofthe developed surface profile measuring instrument for large mirrors is verified. Theresults indicated that the developed instrument can fulfill the demands of non-contact,high-precision and wide measuring range for large mirrors surface profile measurement.Meanwhile, the measurement uncertainty of the developed instrument is evaluated byusing the above-mentioned vector model, and the result shows that the measurementuncertainty of the instrument can be U95≈0.2951μm (k=2).In summary, aimed at the special request for large mirrors in surface profilemeasurement, the thesis propose a novel method which can automatically separate the spindle spatial rotation error and surface profile data, and develop a novel non-contactsurface profile measuring instrument with high-precision and wide measuring range. Thedeveloped method and instrument have great application significance in surface profilemeasurement for large mirrors.